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Used by New Zealand Emergency Services Map Reading Guide 1:50 000 How to use a Topographic map
Used by New ZealandEmergency Services
Map Reading Guide1:50 000How to use a Topographic map
THiS GUidE pRovidES iNfoRMaTioN oN:
• datums
• projections
• theNewZealandtopographicmapseriesTopo50(andhowtoreadthem),and
• howtonavigateusingamap.
FurtherdetailedinformationondatumsandprojectionsinNewZealandcanbefoundin‘Where in the World are We: a Technical Guide to datums and projections in New Zealand’,availableontheLandInformationNewZealandwebsite www.linz.govt.nz/surveypublications.
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Topo50 Map Reading Guide
CoNTENTS
Who makes topographic maps? ........................................................................................................................2
What is a topographic map?...............................................................................................................................2
Creating a map ......................................................................................................................................................3
Geodetic datums ...................................................................................................................................3
Map projections ....................................................................................................................................4
The parts of a Topo50 map ..................................................................................................................................5
Cover and back panel ...........................................................................................................................6
Datum and projection information panel ..........................................................................................7
How to read a Topo50 map ..................................................................................................................................8
Map scale ...............................................................................................................................................8
Distance ................................................................................................................................................10
Directions .............................................................................................................................................10
Bearings ...............................................................................................................................................11
Map symbols (the legend) .................................................................................................................12
Contour lines ........................................................................................................................................13
Relief shading ......................................................................................................................................14
Map coordinates .................................................................................................................................15
Geographical coordinates - latitude and longitude ......................................................................15
Grid coordinates - eastings and northings .....................................................................................16
How to quote a grid reference for a particular point ....................................................................17
planning a trip .....................................................................................................................................................19
Using a GpS .........................................................................................................................................................19
Using GPS with a map ........................................................................................................................20
The magnetic compass......................................................................................................................................21
Compass errors ...................................................................................................................................21
Features of a compass .......................................................................................................................22
Using your compass to reach a destination ...................................................................................22
Conversion of bearings ......................................................................................................................24
Simple uses of a map .........................................................................................................................................24
Orienting a map ...................................................................................................................................24
Finding your present position............................................................................................................25
Setting a course ..................................................................................................................................26
Glossary................................................................................................................................................................28
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Topo
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uide WHo MaKES TopoGRapHiC MapS?
NewZealandTopo50mapsareproducedandpublishedbyLandInformationNewZealand(LINZ).
Topo50mapsareatascaleof1:50 000andshowgeographicfeaturesindetail.Theyareusefulforawiderangeofactivitiessuchaslocalnavigationbyvehicleoronfoot,localityareaplanningandstudyoftheenvironment.
LINZalsoproducessmallerscalemapsat1:250 000,1:500 000,1:1 million,and1:2 millionscales.Thesemapsareusefulforplanningtraveloverlargedistances,orforgivinganoverviewofNewZealand.
Mapsareusedbyawidevarietyofgroupsincludingthemilitary,emergencyservices,andrecreationaluserssuchastrampers.
WHaT iS a TopoGRapHiC Map?
Topographicmapsaredetailed,accurategraphicrepresentationsoffeaturesthatappearontheEarth’ssurface.Thesefeaturesinclude:
• Cultural: roads,buildings,urbandevelopment,railways,airports,namesof placesandgeographicfeatures
• Hydrography: lakes,rivers,streams,swamps,tidalflats
• Relief: mountains,valleys,slopes,depressions
• vegetation: woodedandclearedareas,vineyardsandorchards.
Thelevelofdetailshownonamapdependsonthescaleofthemap;smallscalemapsarelessdetailedthanlargerscalemaps.
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Topo50 Map Reading Guide
CREaTiNG a Map
Portrayingfeaturesonthecurvedsurfaceoftheearthontoaflatmaprequirestheuseofageodeticdatumandmapprojection.
Geodetic datumsMappingandcoordinatesystemsarebasedonageodeticdatum,whichisamathematicalsurfacethatbestfitstheshapeoftheEarth.NewZealand’spreviousdatum-NewZealandGeodeticDatum1949(NZGD49)-wasdefinedin1949andbestfittedtheshapeoftheEarthintheNewZealandregiononly.
In2000,anewnationalgeocentricdatumwasadoptedinNewZealand-theNewZealandGeodeticDatum2000(NZGD2000).ThisdatumisbasedonamathematicalsurfacethatbestfitstheshapeoftheEarthasawhole.ItsoriginisattheEarth’scentreofmass,hencetheterm‘geocentric’.ThedatumalsoincorporatesadeformationmodelusedtomanagedeformationacrossNewZealandasaresultofplatetectonics.Formostuserstheseeffectscanbeignored.
TheprimaryreasonforthechangefromNZGD49toNZGD2000isthewidespreaduseofGlobalSatelliteNavigationSystems(GNSS)suchastheGlobalPositioningSystem(GPS).ThisisbasedonageocentricdatumknownastheWorldGeodeticSystem1984(WGS84).TheTopo50mapseriesusesNZGD2000.Formostpracticalpurposes,WGS84andNZGD2000coordinatesarethesame.
Onamap,datumcoordinatesareexpressedintermsoflatitudeandlongitude.Theseareoftenreferredtoasgeographicalcoordinates.
AsignificantimplicationofthechangefromNZGD49toNZGD2000isthatlatitudeandlongitudevaluesdifferfromtheirNZGD49predecessorsbyapproximately190mNand10mE.Whilefeaturesonthegroundwillnotmove,theircoordinateswillchangebyapproximately200minanortherlydirectionwhenmovingtotheNZGD2000datum.
HiNT: Remember, if you’re using a map that is in terms of NZGd49 your GpS may show your location as being approximately 200m different
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AmapprojectionenablesthecurvedmathematicalsurfaceapproximatingtheEarthtoberepresentedonaflatsheetofpaper(i.e.amap).Manyprojectionscanbedefinedintermsofaparticulargeodeticdatum.
Theprojectionprocessresultsinthemap’sspatialrepresentationbeingdistorted.Imaginestretchingandtearingabasketballtomakeitscurvedsurfacelieflatontheground.Themagnitudeofthedistortioncanbecalculated,allowingcorrectionstobemadewhennecessary.
Therearemanydifferenttypesofprojection,eachhavingitsownadvantagesanddisadvantages.Noprojectionisperfect.Theprojectionchosenforamapwillhaveminimal,oracceptable,distortionrelativetothemap’sscaleandpurpose.
Mapprojectionsgenerallyusearectangulargridcoordinatesystem.Thesegridcoordinatesaredescribedintermsofeastingandnorthing,thedistanceseastandnorthofanorigin.Theoriginisassignedasetofcoordinatesandthisisoftentermedthefalseorigin.Gridcoordinatesareusuallyexpressedinunitsofmetres.
TheTopo50mapseriesusestheNewZealandTransverseMercator2000(NZTM2000)projection.Theprevious1:50000NZMS260mapseriesusedadifferentprojectioncalledtheNewZealandMapGrid(NZMG).
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Topo50 Map Reading Guide
THE paRTS of a Topo50 Map
ThetwomainpartsofaTopo50mapare:
• themapface,whichshowstheareamappedandincludesinformationtohelpyouvisualiseorrecognisetheareaandlocatefeaturesonthemap;and
• themapmargininformation,whichgivesdetailstohelpyouusethemap,aswellasexplanationsonwhen,whereandhowtheinformationwascompiled.
Topo50 Map of CHRiSTCHURCH (BX24)
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OnaTopo50mapacoverpanelshowsthemapsheetnameandindicatestheareaofthemapandsurroundingmapsheets.
Thebackpanelshowsthegenerallocationofthemap,thepublicationdateofthemap,wherefurtherinformationcanbefound,andimportantlimitationswithinformationshownonthemap.
Mapsareproducedfrominformationavailableonacertaindate.Overtime,thatinformationmaychange.TheTopo50mapshaveapublisheddate.TherewillbemapsontheLINZwebsiteatwww.linz.govt.nzshowingthedateparticularareaswerelastmaintainedsouserscanseetheageofthedataonanyparticularmap.
fRoNT aNd BaCK CovER paNElS of Topo50 Map of CHRiSTCHURCH (BX24)
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Topo50 Map Reading Guide
datum and projection information panelInformationonthedatumandprojectionusedareshownontheinformationpanelofaTopo50map.
dETailS pRovidEd oN a Topo50 Map
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Thefirststepinreadingatopographicmapistobecomefamiliarwiththespecificcharacteristicsofthemapormapsyou’reusing.
Openupyourmap,checkitcoverstheplacesofinterestandthenfindthefollowingcharacteristics:
HiNT: pay attention to how your map unfolds so you can fold it up again correctly
Map Scale• What is the map scale?Amaprepresentsagivenareaontheground.Amapscalereferstotherelationship(orratio)betweendistanceonamapandthecorrespondingdistanceontheground.Themapscaletellsyouaboutthecomparativesizeoffeaturesanddistancesdisplayedonthemap.
• Which direction is north?Thisisimportantbecausethenorthpointorientsthemaptotherealworld.
• What symbols are used on the map?Havealookatthelegend.Tounderstandthemapyouneedtounderstandthesymbolsused.Featuresthatappearonmapswithdifferentscalesmaybedepictedbydifferentsymbols.
What datum and projection is used?Ifyouaregoingtousethecoordinatesfromthemap,youwillneedtodeterminewhichcoordinatesystem(ordatum)andprojectionisusedonthemap.Alwaysincludeareferencetothedatumorprojectionwhenquotingcoordinates.Datumsandprojectionsareexplainedearlierinthisbooklet.Thisinformationwillbecontainedinthetextonthemapfooter(seeprevioussection‘Datumandprojectioninformationpanel’).
Ifyou’reusingaGlobalNavigationSatelliteSystem(GNSS)suchastheGlobalPositioningSystem(GPS),remembertosetyourGNSSreceivertothesamecoordinatesystemasyourmap,oracompatibleone.MapsonNewZealandGeodeticDatum2000(NZGD2000)datumarecompatiblewithWGS84usedinGNSS.
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Topo50 Map Reading Guide
TheTopo50mapsareatascaleof1:50000.CommonscalesforNewZealandtopographicmapsare:
Scale Ground distance of 1cm on the map
Larger
Smaller
1:50000 500m
1:250000 2.5km
1:1million 10km
1:2million 20km
Toexplainscalesgraphically,lookata1:50000scaleTopo50map.Thefirstnumberofthescale(1)representsacoreunitofdistanceonthemap,whilethesecond(50000)representsthatsamedistanceontheground.
Inthiscase,onecentimetreonthemaprepresents50000centimetres,or500metres,ontheground.ThedistancebetweenTrigsA582andMQZGonthefollowingmapismeasuredat9cmatthemapscale,whichequatesto4.5kmontheground.
EXaMplE of SCaliNG diSTaNCE of a Topo50 Map (NoT To SCalE)
Thelargerthescaleofamap,thesmallertheareathatiscoveredandthemoredetailedthegraphicrepresentationoftheground.Forexample,smallscalemaps(suchas1:250000)aregoodforlongdistancevehiclenavigation,whilelargescalemaps(1:50000)areidealfortravelonfoot.
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Inadditiontothemapscale,mostmapsalsoshowascalebar:
SCalE BaR foR a Topo50 Map (NoT To SCalE)
Usingthescalebaronamapyoucandeterminethedistancebetweentwopointsonthemap.
HiNT: Use a piece of string, ruler or strip of paper to measure the distance between two points on the map. Then hold the right hand end of the measurement on a whole number of kilometres on the scale so the other end of the measurement is to the left of the 0 marker and note the whole number of kilometres (right hand point measurement). add to this the percentage of a kilometre read to the left of the zero marker
directionsMapsusuallyincludeanorthpointdiagraminthemapmargininformation.ThisshowsthedirectionofGridNorthandMagneticNorthatthecentreofthemap.
EXaMplE of a NoRTH poiNT diaGRaM oN a Topo50 Map
• True North(TN)isthedirectiontotheEarth’sgeographicNorthPole.
• Grid North(GN)isthedirectionoftheblueverticalgridlines(eastings)onaTopo50map.TheangulardifferencebetweenGNandTNisknownasgridconvergence.
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Topo50 Map Reading Guide
• Magnetic North(MN)isthedirectionfromanypointonthesurfaceoftheEarthtowardstheEarth’sNorthMagneticPole.TheangulardifferencebetweenTNandMNisknownasMagneticDeclination.AsGNisusedinpreferencetoTNformapreadingpurposes,itismoreusefultoknowthedifferencebetweenGNandMN.ThisisknownastheGrid/Magneticangle.ThisvariesacrossNewZealandandbecausethepositionoftheNorthMagneticPolemovesslightlyfromyeartoyear,theGrid/MagneticangleandMagneticDeclinationwillvarybyasmallamounteachyear.Inusingamapforaccuratenavigation,magneticvariationcanbeimportant,particularlyifthemapisseveralyearsold.
BearingsDirectionscanalsobeexpressedasbearings.Abearingistheclockwisehorizontalanglemeasuredfromnorthtoachosendirection.Bearingsareusuallyshownindegreesandrangefrom0°(north)to360°(alsonorth).Southis180°,eastis90°,westis270°.Bearingsareoftenusedfornavigatingbetweenpoints.
illUSTRaTioN dEpiCTiNG BEaRiNGS of 40° aNd 320°
a CoMpaSS RoSE aNd BEaRiNG GUidE
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Mapsusesymbolstorepresentfeaturesontheground.Thesefeaturesincluderoads,tracks,rivers,lakes,vegetation,fences,buildings,powerlinesetc.Giventhesizeofamap,itisnotpossibletoshowallfeaturesontheground.Largescalemapsshowmoredetailandalargernumberoffeatures.Dependingonthescaleofthemaps,featuresmayhavetobeoffsetsotheycanbeclearlyshownonthemap,e.g.aroadwayandarailwaylinemayhavetobeseparatedhorizontallysotheydon’toverlayeachother.
Colourplaysanimportantpartinsymbolsandsomeinternationalconventionsapplytotheuseofcolour.Forexample,blueforwaterfeatures,blackforcultureandgreenforvegetation.
Symbolsaregroupedinthemesonthelegend.Whilemostsymbolsareeasilyrecognisedasthefeaturestheyrepresent,youcanalwaysrefertothemap’slegend.
paRT of a lEGENd fRoM a Topo50 Map
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Topo50 Map Reading Guide
Contour linesTopo50mapsshow20mcontourlines.Theselines,whichjoinpointsofequalheight,representthereliefintheterraindepicted.Forexample,iftherearemanycontourlinesclosetogether,theterrainissteep.Contourlinesthatarefarapartindicatelandwithgentleslopes.Theaccuracyofacontourlineisusuallytakentobeabouthalfthecontourinterval,i.e.10monaTopo50map.
Thecoastlineonamaprepresentsthelineofmeanhighwaterlevel(MHW).However,contoursrepresenttheheightsabovemeansealevel(MSL).
HiNT: Contour values read uphill. as you read the contour numbers, you will be looking up hill
EXaMplE of CoNToUR SHapES (NoT To SCalE)
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Inadditiontocontourlines,reliefshadinghelpsyouvisualisetheterrain.Hillsandvalleysareshadedasiftheywereilluminatedfromthenorth-west.
EXaMplE of REliEf SHadiNG (NoT To SCalE)
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Topo50 Map Reading Guide
Map coordinatesMapcoordinatesareusuallyshowninoneoftwoways:
• geographical coordinates,givenaslatitudeandlongitudevaluesindegrees,minutesandseconds,e.g.NewZealandGeodeticDatum2000geographicalcoordinates;or
• grid coordinates,givenaseastingandnorthingvalues,inmetres,e.g.NewZealandTransverseMercator2000projectiongridcoordinates.
Topo50mapsshowageographicgraticule(latitudeandlongitude,indegrees,minutesandseconds)andacoordinategrid(eastingsandnorthings,inmetres),soyoucandeterminerelativeandabsolutepositionsofmappedfeatures.
EXaMplE of Map CooRdiNaTES USEd oN a Topo50 Map (NoT To SCalE)
Geographical coordinates - latitude and longitudeYoucanfindorexpressalocationusingthegeographiccoordinatesoflatitude(northorsouth–horizontallines)andlongitude(eastorwest–verticallines).
Latitudeistheangularexpressionofthedistancenorthorsouthfromtheequator(0°latitude).TheSouthPoleisat90°S;theNorthPoleat90°N.
LongitudeistheangularexpressionofthedistanceeastorwestfromtheimaginarylineknownasthePrimeMeridianat0°longitude.
Geographicalcoordinatesaremeasuredindegrees(°),minutes(‘)andseconds(“).Eachdegreeisdividedinto60minutes;eachminuteisdividedinto60seconds.Whenexpressingcoordinates,
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followingwouldbestatedas:43°44’55”S172°37’00”E.
OntheTopo50mapsthelatitudeandlongitudecoordinatesareshownalongtheedgesofthemapfaceasblacklineswithshortblackmarkersthatindicatetheminutesoflatitudeandlongitude.
WaRNiNG: The lines of latitude and longitude which join the black tick marks on Topo50 maps are not parallel to the grid because of the effects of the Earth’s curvature and projection used
EXaMplE of laTiTUdE aNd loNGiTUdE liNES aloNG THE EdGE of a Topo50 Map (NoT To SCalE)
Grid coordinates - eastings and northingsGridlinescanalsobeusedtofindorexpressalocation.Gridlinesaretheequallyspacedverticalandhorizontalintersectinglinessuperimposedovertheentiremapface.Eachlineisnumberedattheedgeofthemapface.On1:50000Topo50maps,thedistancebetweenadjacentlinesis2cmwhichrepresents1000metres.
OfficialLINZmapsareprintedsogridnorthpointstothetopofthesheet.Onesetofgridlinesrunsnorth-south,whiletheotherrunswest-east.Thepositionofapointonthemapisdescribedasitsdistanceeastfromanorth-southlineanditsdistancenorthofaneast-westline.
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Topo50 Map Reading Guide
Forthisreason,gridlinesarealsocalled:
• eastings–thesearetheverticallinesrunningfromtoptobottom(northtosouth).Theydividethemapfromwesttoeast.Theirvaluesincreasetowardstheeast;and
• northings–thesearethehorizontallinesrunningfromlefttoright(westtoeast).Theydividethemapfromnorthtosouth.Theirvaluesincreasetowardsthenorth.
Thesquaresformedbyintersectingeastingsandnorthingsarecalledgridsquares.OnTop50mapseachsquarerepresentsanareaof100hectaresoronesquarekilometre.
How to quote a grid reference for a particular pointAgridreferenceisusedtodescribeauniquepositiononthefaceofthemap.Thedegreeofaccuracyrequiredwilldeterminethemethodusedtogenerateagridreference.Allmethodsfollowasimilarapproach.ThetwotablesbelowdescribehowtogiveasimplifiedsixfiguregridreferenceandafullnorthingandeastinggridcoordinateforMtCavendishshownonTopo50mapBX24.
Thesimplifiedwayofexpressingthefullgridcoordinateistospecifythesheetnumberandasixfiguregridreference.Remembertoquotethesheetnumberasthesixfiguregridreferenceisnotuniquetoasinglesheet.
HoW To REad a SiX fiGURE GRid REfERENCE oN a Topo50 Map foR MT CavENdiSH
HiNT: if a grid reference starts with a zero, remember to include it
Afullgridcoordinatetothenearest100misgivenbyasevenfigureeastingandnorthingfortheexampleabove.
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To GivE aN EaSTiNG aNd NoRTHiNG GRid CooRdiNaTE oN THiS Map(Notethatthisexamplegivesthecoordinatetothenearest100metres)
SaMplE poiNT Mt Cavendish
East Coordinate
1.Recordthefirsttwodigitsofthefulleasting(E)giveninthebottomleftcornerofthemapmargin.Thesearethefirstandsecondnumbersoftheeastinggridcoordinate.
2.LocatethefirstVERTICALgridlinetotheLEFTofthesamplepoint.
3.Readthegridvalueslabellingthelineineitherthetoporbottommarginoracrossthemiddleofthemap.Thesearethethirdandfourthnumbersoftheeastinggridcoordinate.
WaRNiNGIfthesenumbersarelessthanthethirdandfourthnumbersinthebottomleftcornervaluesofthemap,add1tothefirsttwodigits.
4.Estimatetenthsofagridsquareeastwardfromthegridlinetothepoint.Thisisthefifthnumberofthefulleastinggridcoordinate.
5.Thefinaltwonumbersofthefulleastinggridcoordinateare00aswecanonlyestimatethereferencetothenearest100metres.
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1
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1577100
North Coordinate
1.Recordthefirsttwodigitsofthefullnorthing(N)giveninthebottomleftcornerofthemapmargin.Thesearethefirstandsecondnumbersofthenorthinggridcoordinate.
2.LocatethefirstHORIZONZTALgridlineBELOWthesamplepoint.
3.Readthegridvalueslabellingthelineineithertheleftorrightmarginofthemap.Thesearethethirdandfourthnumbersofthenorthinggridcoordinate.
WaRNiNGIfthesenumbersarelessthanthethirdandfourthnumbersinthebottomleftcornervaluesofthemap,add1tothefirsttwodigits.
4.Estimatetenthsofagridsquarenorthwardfromthegridlinetothepoint.Thisisthefifthnumberofthefullnorthinggridcoordinate.
5.Thefinaltwonumbersofthefullnorthinggridcoordinateare00aswecanonlyestimatethereferencetothenearest100metres.
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73
7
00
5173700
SaMplE CooRdiNaTE 1577100 mE 5173700 mN
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Topo50 Map Reading Guide
plaNNiNG a TRip
Planningasuccessfulroutethroughroughcountryusuallyrequiresatopographicmap,acompass,perhapsaGlobalNavigationSatelliteSystem(GNSS)receiversuchasaGlobalPositioningSystem(GPS)receiver,andobservationofvariouslandforms.Streamsandvegetationcanhelpwithnavigationbutmayhinderyourprogress.
Makesureyouhavetherightscalemapforthetripyouareplanning.Obviously,journeysonfootshouldbesupportedbyalargerscalemap,orsetofmaps.
Often,routefindingdoesnotrequiregreataccuracy,butitdoesrequireplanning.Beforesettingout,studythemap.Findyourstartandfinishpoints.Theterraindepictedonthemapwillhelpyouselectasuitableroute,andanticipateandmakebestuseofthefeaturesyouwillencounter.
Forexample,youmaydiscoveraleadingspurormainridgethatwillhelpyouavoidarivervalleywithcliffsorsteepterrain.Youwillalsobeabletomeasuretheroute’sdistanceandanyheightstoclimb,allowingyoutoestimatehowlongeachstageofthetripwilltake.
USiNG GpS
TheGPSisoneofanumberofGNSSandhasbeendevelopedbytheUSA’sDepartmentofDefense.Itiswidelyusedforciviliannavigationandpositioning,surveyingandscientificapplications,andalthoughanexcellenttool,itisbestusedwithamap.
GPSreceivershavemanyusefulfeaturesfornavigation,suchastheabilitytostorepositionsanddeterminespeedanddirectionoftravel(whicharebeyondthescopeofthisguide).Provideditisusedcorrectly,acomparativelyinexpensive,hand-heldGPSreceivercanprovidepositionswithaccuracybetterthan15mandoftenatthe5mlevel.
NoTE: a GpS is no substitute for a map and compass
EXaMplES of GpS RECEivERS
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AGPSreceivercalculatespositionbymeasuringdistancestofourormoreGPSsatellites.GPSisaccessible24hoursaday,anywhereintheworld,inallweather.
GPSisbasedontheWGS84datum(seeexplanationofdatumsonpage3).
ForpracticalpurposesNZGD2000canberegardedasthesameasWGS84.Itisimportanttocheckwhichdatumyourmapisbasedupon.
HiNT: Set your GpS datum to match your map datum
Thisdatuminformationwillbeshowninthemapmargin.ForthebestmatchbetweenyourmapcoordinatesandGPSreceiver,configuretheGPSreceivertodisplaycoordinates(geographicalorgrid)onthesamedatumasthemapbeingused.
MostGPSreceivershavetheabilitytodisplayeithergeographicorgridcoordinatesonanumberofnationalandregionaldatums.Itisimportanttoknowhowtosetthecorrectdatuminyourreceiver.PleaseconsultyourGPSreceiver’suserguidefordetails.Ifthedatumyouneedisnotofferedinyourreceiver,consultyourGPSdealerforassistance.
ItisrecommendedpracticetocheckyourGPSreceiveragainstwell-definedmapfeatureseverytimeyouuseit.Visitafeaturesuchasaroadintersection,determineitspositionbyGPSandcomparethiswithcoordinatescalculatedfromamap.Thelargerthescaleofthemapthebetter.Thecoordinatesofsurveycontrolmarksortrigpoints,maybeobtainedfromtheLINZgeodeticdatabaseatwww.linz.govt.nz/gdb
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Topo50 Map Reading Guide
THE MaGNETiC CoMpaSS
Amagneticcompassisanimportantaidtoroute-findingandanyonewhoventuresintotheoutdoorsshouldcarryone.
Acompassworksontheprinciplethatthepivotingmagnetisedneedle(orthenorthpointoftheswingingdial)alwayspointstothenorthmagneticpole.
Asaresult,youcanuseacompasswithgraduations(degrees)markedonittomeasurethebearingofachosendirectionfrommagneticnorth.
HiNT: Metal objects such as cars, fence posts and wires, steel power poles and transmission lines, can affect the accuracy of a compass reading. Stand clear of such items when using a compass – at least 1m from metal fence posts and wires and up to 20m from a car
Compass errorsGeologicalfeaturessuchasironoredepositsanddoleriterockthathasbeenstruckbylightningcanaffectacompass.Itisevenpossiblefortheneedletobecomereverse-polarisedifitisstoredforalongtimenearastronglymagnetisedobject.
Itisthereforeadvisabletotreatmagneticbearingswithcautionandtochecktheaccuracyofyourcompass.Determinemagneticbearingsbetweenobjectsatleastonekilometreapart,usinginformationavailablefromamapandcomparethemwithyourcompassbearing.Thisshouldberepeatedindifferentdirections.
Checkforlocalanomaliesbyreadingbearingsbetweenobjectsabout100metresapartinoppositedirections–thebearingsshoulddifferby180degrees.
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Therearemanytypesofcompasses.Thepivotedneedle,adjustabledialcompassisthemostusefultype.Seeexample–Silvacompassbelow.
Aswellasanorth-pointingNeedle,itwilloftenhaveatransparentbasewithaDirectionofTravelArrowandOrientingLinesmarkedontheRotatingDialhousing,soitcanbeusedasaprotractorformeasuringgridbearingsonamap.
fEaTURES of a TypiCal CoMpaSS
Using your compass to reach a destinationTofollowcompassbearingstoyourchosendestination,youwilleitherneedtodeterminemagneticbearingstovisiblefeaturesalongtheroute,orwillalreadyhavethesebearings.
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Topo50 Map Reading Guide
To dETERMiNE MaGNETiC BEaRiNGS:
1Selectavisiblefeaturealongtherouteyouwanttotravel.Holdingthecompasslevel,pointtheDirectionofTravelArrowatthevisiblefeature.
2FindyourbearingtothevisiblefeaturebyturningtheCompassDialuntilthe“N”alignswiththeredendoftheNeedle.ReadyourbearingindegreesattheIndexLine.
3KeepingtheNeedlealignedwiththe“N”,proceedinthedirectionindicatedbythebearingattheIndexLine.Thebearingwillhelpyoukeepontrackwhenthefeatureisnotvisible.Repeatthisprocedureuntilyoureachyourdestination.
WHEN MaGNETiC BEaRiNGS aRE KNoWN:
1Ifyou’vebeengivenabearingindegreestotravel,turnthedialsothatthebearingissetattheIndexLine.Holdthecompasslevelinfrontofyou,withtheDirectionofTravelArrowpointingstraightahead.
2TurnyourbodyuntiltheredendoftheNeedleisalignedwiththe“N”onthedial.Youarenowfacingyourdirectionoftravel.
3Pickoutavisiblefeatureinlinewithyourbearingandwalktoit.Repeattheprocedureuntilyoureachyourdestination.
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Magneticbearingsmeasuredwithacompassmustbeconvertedtogridbearingsforplottingonamap.Similarly,gridbearingsmeasuredonamapmustbeconvertedtomagneticbearingsforcompassnavigationontheground.
Thegrid/magneticangleisthedifferencebetweengridnorthandmagneticnorthandisapositivevalueifmagneticnorthitiseastofgridnorthandanegativevalueifitiswestoftruenorth.
Toconvertfromamagneticbearingtoagridbearing,addthegrid/magneticangletothemagneticbearing.Toconvertagridbearingtoamagneticbearing,subtractthegrid/magneticangle(seepage10).
HiNT: G M S rule is: Grid to Magnetic Subtract (Good Morning Sunshine)
Eg,GridBearing62OGrid/Magneticangle-20OMagneticBearing=42O
SiMplE USES of a Map
orienting a mapItisagoodhabittoorientyourmapbeforereadingit.Todothis,holdyourmaphorizontallyandrotateituntilitsdirectionandfeaturescorrespondtowhatyouseebeforeyouontheground.
Ifyouareunabletoidentifythesurroundingfeatures,youcanusethecompasstoorientthemap.Todothis:
• Holdthemapflat.Placethecompassonthemapsothatthelongedgeofthebaseplate,oralineintheadjustabledial,sitsoverorisparalleltoanorth-southgridline.Ensurethatthe‘N’onthedialpointstonorthonthemap.
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• Turnthemapwiththecompassonituntilthemagneticneedlepointstothemagneticvariation(whichisapproximately20°currently).
Oncethemapisoriented,youshouldbeabletoidentifyprominentfeaturesinthelandscape.
finding your present positionIfyouhaveaGPSreceiver,youcanuseittodetermineyourcoordinates,rememberingtosetittoadatumcorrespondingtothedatumonyourmap.Or,onceyoucanidentifysurroundingfeaturesonthegroundandonthemap,youcanusethefollowingproceduretofindyourcurrentposition.
HiNT: pack your map and compass in an easy-to-reach place. in wet weather, put the map, with the appropriate area displayed, in a clear plastic bag
1. Choosetwovisiblefeaturesandfindtheseonyourmap.NowpointtheDirectionofTravelArrowtowardsonefeatureandrotatetheCompassDialuntiltheredendoftheNeedlepointstothe“N”onthedial.
2. Addthegrid/magneticangletothebearingshownattheIndexLineandturnthedialtothenewbearing.
3. PlacethecompassonyourmapwiththesideedgeofthebaseplatetouchingthefeatureandpivotituntiltheOrientingArroworlinesalignwiththegridnorthlines.Drawalinefromthefeaturealongthesideofthebaseplateacrossthemap.
4. Repeatthisprocesswiththesecondfeature.Yourlocationiswherethetwolinesintersect.
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Onceyouhaveorientedyourmapandidentifiedyourposition,youcansetacourse.Dothisbysightingorbylayingastraightline(usingtheedgeofthemapcardorapieceofstring)acrossthemap.Itisalsogoodpracticetoidentifyadistantvisiblefeatureontheline,suchasarockyoutcrop,andproceed.Thenidentifyanotherfeatureonthelineandsoonuntilyoureachyourdestination.
Whenfeaturesaresparse,youcoulduseaGPSreceiver.First,determinethecoordinatesofthedestinationpointfromthemapandenterthemintothereceiver,thenwalkintheapproximatedirectionofyourdestination,lettingthereceiverpointyouintherightdirectionasyougo.
HiNT: Check your map to determine if there are land features that may prevent you from following your GpS bearing
oR yoU CaN USE yoUR Map aNd CoMpaSS iN THiS Way:
1Beforeyoustartonyourway,placethecompassonthemapsothatthesideedgeofthebaseplateconnectsyourpresentpositiontoyourdestinationandtheDirectionofTravelArrowisalsopointingthatway.
2TurnthecompassdialuntiltheOrientingLinesareparallelwiththegridnorthlinesonthemapandtheOrientingArrowisalsopointingtogridnorth.
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Topo50 Map Reading Guide3Thedial’sreadingattheIndexLineshowsthegridbearing.SubtracttheGrid/Magnetic anglefromthisbearingandturnthedialtoshowthenewmagneticbearingattheIndexLine.
4Putthemapaside.HoldthecompasssteadyandlevelinfrontofyouwiththeDirectionofTravelArrowpointingstraightahead.TurnyourbodyuntiltheredendoftheNeedleisdirectlyovertheOrientingArrow,pointingtothe“N”onthedial.TheDirectionofTravelArrownowpointstoyourdestination.Lookup,aligntheDirectionofTravelArrowwithafeatureandwalktoit.Repeatthisprocedureuntilyoureachyourdestination.
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Bearing–geographicorientationofalinegivenasananglemeasurementindegreesclockwisefromtruenorth.
Cartography–theartandscienceofproducingmaps,chartsandotherrepresentationsofspatialrelationships.
Contour–alinedrawnonamapjoiningallthepointsontheEarththatarethesameheightabovemeansealevel.
Coordinates–angularorlinearvaluesthatdesignatethepositionofapointinagivendatumorprojectionsystem.
Coordinates, geographic–asystemofsphericalcoordinatescommonlyknownaslatitudeandlongitude.
Coordinates, grid–aplane-rectangularcoordinatesystemexpressedaseastingsandnorthings.
datum–amathematicalsurfaceonwhichamappingandcoordinatesystemisbased.
Elevation–theheightabovemeansealevel.
Geocentric datum–adatumwhichhasitsoriginattheEarth’scentreofmass.Theadvantageofthegeocentricdatumisitsdirectcompatibilitywithsatellite-basednavigationsystems.
Geographical coordinates–apositiongivenintermsoflatitudeandlongitude.
GpS–GlobalPositioningSystem–isasatellitebasednavigationsystemdevelopedbytheUnitedStatesDepartmentofDefenseandwidelyusedforciviliannavigationandpositioning.
GNSS–GlobalNavigationSatelliteSystem
Graticule–anetworkoflinesonamaporchartrepresentingtheparallelsoflatitudeandmeridiansoflongitudeoftheEarth.
Grid–twosetsofparallellinesintersectingatrightanglestoformsquares.
Grid convergence–theangulardifferencebetweenGridNorthandTrueNorth.
Grid coordinates–theequallyspacedverticalandhorizontalintersectinglinessuperimposedoverthefaceofamap.Onesetofgridlinesrunsnorth-south,whiletheotherrunswest-east.Thepositionofapointonthemapisdescribedasitsdistanceeastfromanorth-southlineanditsdistancenorthofaneast-westline.
Grid/magnetic (G-M) angle–thedifferencebetweengridnorthandmagneticnorthandisapositivevalueifmagneticnorthitiseastofgridnorthandanegativevalueifitiswestoftruenorth.
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latitude–thelatitudeofafeatureisitsangulardistanceonameridian,measurednorthwardsorsouthwardsfromtheEquator.
longitude–anangulardistancemeasuredeastorwestalongtheequatorfromareferencemeridian(Greenwich).
Magnetic north–thedirectionasindicatedbyacompasstotheearthmagneticpole.
Map–arepresentationoftheEarth’ssurface.Acadastralmapisoneshowingthelandsubdividedintounitsofownership;atopographicmapisoneshowingthephysicalandsuperficialfeaturesastheyappearontheground;athematicmapdisplaysaparticulartheme,suchasvegetationorpopulationdensity.
Map projection–anysystematicwayofrepresentingthemeridiansandparallelsoftheEarthuponaplanesurface.
Mercator projection–theconformalcylindricalprojectiontangentialtotheEquator,possessingtheadditionalvaluablepropertythatallrhumblinesarerepresentedbystraightlines.Usedextensivelyforhydrographicandaeronauticalcharts.
Meridian–animaginarylinefromtheNorthPoletotheSouthPoleconnectingpointsofequallongitude.
NZGd49–NewZealandGeodeticDatum1949-alocaldatumthatwasabestfittotheshapeoftheEarthintheNewZealandregion.IthasnowbeensupersededbyNZGD2000.
NZGd2000–NewZealandGeodeticDatum2000–ageocentricdatumbasedonamathematicalsurfacethatbestfitstheshapeoftheEarthasawhole,withitsoriginattheEarth’scentreofmass.
NZMG–NewZealandMapGrid–aconformalmappingprojectionadoptedforNewZealandin1973withminimalscaleerror.BasedonNZGD49.
NZTM2000–NewZealandTransverseMercatorprojection2000-aTransverseMercatorprojectionbasedonNZGD2000.Theunitofmeasureisthemetre.
Relief–thedeviationofanareaoftheEarth’ssurfacefromaplane.ItreferstothephysicalshapeofthesurfaceoftheEarth.
Rhumb line–acurveonthesurfaceofaspherethatcutsallmeridiansatthesameangle;thepaththatmaintainsaconstanttruebearing.
Topography–descriptionorrepresentationonamapofthephysicalandculturalsurfacefeatures.
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devisedbyGauss,alsoknownastheGauss-Krugerprojection.Asitsnameimplies,itsconstructionisonthesameprincipleastheMercatorprojection,theonlydifferencebeingthatthegreatcircleoftangencyisnowanynominatedmeridian.Meridiansandparallelsarecurvedlines,exceptforthecentralmeridianforaspecifiedzone(meridianoftangency),whichremainsastraightline.Theamountofscaledistortionmaybecomeunacceptableatdistancesgreaterthanabout1.5degreesinlongitudefromthecentralmeridian.
WGS84–WorldGeodeticSystem1984–ageocentricgeodeticdatumdevelopedbytheUnitedStatesDepartmentofDefenseforusewithGPS.Formostpracticalpurposes,NZGD2000isequivalenttoWGS84.
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byGeoscienceAustralia,GPOBox378,Canberra,ACT2609,Australia.ThanksaregiventoGeoscienceAustraliaforpermissiontousethisinformation.
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Wewelcomeadviceoferrors,omissionsorchangesuggestions:[email protected]“Topo50MapFeedback”inyoursubjectline,orphone0800665463.
